The clinical evaluation of an infant with motor delay, muscle weakness, and/or hypotonia would improve considerably if muscle strength could be measured objectively and normal reference values were available. The authors developed a method to measure muscle strength in infants and tested 81 typically developing infants, 6-36 months of age, and 17 infants with Prader-Willi Syndrome (PWS) aged 24 months. The inter-rater reliability of the measurement method was good (ICC = 0.84) and the convergent validity was confirmed by high Pearson's correlations between muscle strength, age, height, and weight (r = 0.79-0.85). A multiple linear regression model was developed to predict muscle strength based on age, height, and weight, explaining 73% of the variance in muscle strength. In infants with PWS, muscle strength was significantly decreased. Pearson's correlations showed that infants with PWS in which muscle strength was more severely affected also had a larger motor developmental delay (r = 0.75). (Contains 4 figures and 3 tables.)